In electronic circuits, where two or more supplies are needed to operate the circuit and it is required to switch between the multiple supplies, it is typically necessary to assure a switch-over which does not lead to glitches. This requirement becomes more important in case of jitter-free, highly stable circuits. For example, in a frequency generating system, with high stability (minimum variation in frequency), where a regulator is being used to supply a voltage to an oscillator circuit, a smooth switch over is crucial.
However, in such circuits, variation in a core supply (VddCore) of a system-on-a-chip (SoC) typically introduces variation in frequency, and noise due to the high SoC current often results in jitter in the RC clock. Usually, to achieve a 1 microsecond (μs) start-up time, the oscillator can be started with ‘VddCore’ first. Subsequently, the supply voltage to the oscillator must be switched from ‘VddCore’ to a regulator output (VREG), which can provide an accurate frequency as desired. The switch-over from ‘VddCore’ to VREG should therefore be smooth (e.g. no overshoot/undershoot) and clean, to prevent any variation/jitter in frequency due to this switch-over.
Also, with low consumption and high power supply rejection ratio (PSRR) (e.g. −35 dB) criteria, the bandwidth of a conventional bandgap driven regulator system is typically restricted, thus making it a slow responding system. This results in a large settling time to attain the final voltage value when the system is switched from the core supply voltage to the regulated voltage. The large settling time can cause cycle-to-cycle jitter in the on-chip oscillator.
For example, VddCore can vary from 1.08 Volts (V) to 1.32V, while VREG remains stable at 1.2V. Thus, switching the supply or the load from 1.08V (or from 1.32V) to 1.2V may force the load to be suddenly provided with 1.2V instead of 1.08V (or 1.32V). Thus, this transition may become jittery and un-controlled.
A need therefore exists to provide a system and method for switching from a SoC core supply to a regulated supply that seek to address at least one of the above problems.